feat(coreos-base/oem-pxe): introduce the PXE oem

simple OEM for PXE. Simply has an oem-release file.
feat(coreos-base/coreos-init): add ssh-key-proc-cmdline

adds the sshkey= feature on the kernel commandline
Merge pull request #172 from philips/pxe-initial

introduce the PXE oem